Backside Illumination (BSI) image sensor chips are replacing front-side illumination sensor chips for their higher efficiency in capturing photons. In the formation of the BSI image sensor chips, image sensors, such as photo diodes, are formed at a front surface of a silicon substrate of a BSI wafer. The logic circuits for processing the signal of the image sensors are also formed at the front surface of the silicon substrate. An interconnect structure is then formed over, and connect to, the image sensors and the logic circuits.
After the formation of a BSI wafer, the BSI wafer is bonded to a carrier, and the silicon substrate is thinned. As a result, the silicon substrate is thin enough for light to penetrate through the silicon substrate from the backside. During and after the thinning, the carrier provides mechanical support to the thin wafer. Additional components such as metal grid, color filters, micro-lenses, and the like, are then formed on the BSI wafer. The BSI wafer and the carrier are then sawed apart into BSI chips.